A method and apparatus are disclosed for high speed multiple-access data communication. High speed optical guided wave components and angular division multiplexing are used to obtain parallel optical signal transmission and processing in routing data packets between host computers.
Multiple-access data networks have been realized in the prior art using multimode optical fiber at data rates up to 100 Mbits/s. See, for example, E. Rawson and R. Metcalfe, "Fibernet: Multimode Optical Fibers for Local Computer Networks," IEEE Trans. Comm., COM-26, No. 7, 983-990 (July 1978); E. G. H. Lean, "Multimode Fiber Devices for Optical Fiber Links, Printing, and Display," Ibid., 962-967; J. D. Crow, "(GaAl)As Laser Requirements for Local Attached Data Link Applications," IBM Journ. of Res. and Div., 23, No. 5, 576-584 (September 1979). In such networks, passive components of low insertion loss, e.g., biconically-tapered, reflective, and transmissive star configurations have been used to perform various data distribution functions. See, for example, B. S. Kawasaki and K. O. Hill, "Low-Loss Access Coupler for Multimode Optical Fiber Distribution Networks," Appl. Opt., 16, No. 7, 1794-95 (July 1977); M. Hudson and F. Thiel, "The Star Coupler: A Unique Component for Multimode Optical Waveguide Communications Systems," Appl. Opt., 13, No. 11, 2540-2545 (Nov. 1974); T. Ozeki and B. S. Kawasaki, "New Star Coupler Compatible with Single Multimode-Fiber Data Links," Electronics Lett., 12, No. 6, 151-152 (March 1976); M. K. Barnoski, "Design Considerations for Multiterminal Networks," in Fundamentals of Optical Fiber Communications (Ed. M. K. Barnoski) (Academic Press, New York, 1976). These networks are effective for interconnecting computers and other data equipment operating at speeds consistent with present technology, i.e., 30-50 nanosecond memory cycle times. However, the situation will change rapidly when higher-speed computers with memories approaching one nanosecond cycle times, such as a cryogenic memory composed of Josephson junctions capable of six picosecond switching times, are available for connection to data networks. Optical data networks will then have to operate efficiently at the Gbit/sec. information-carrying capacity of high quality optical fiber. See T. Li, "Optical Fiber Communication--The State of the Art," IEEE Trans. Comm., Com-26, No. 7, 946-955 (July 1978).
Extremely high-speed computer networks will employ guided optical wave components such as modulators, switches, logic gates, amplifiers, and couplers fabricated from a wide range of materials. For example, recently reported logic elements with propagation delays in the 20-40 psec./gate range operate at 5 volts, compatible with electronic logic gates (TTL, etc.). H. F. Taylor, "Guided Wave Electrooptic Devices for Logic and computation," Appl. Opt., 17, No. 5, 1493-1498 (May 1978). These elements, assembled to form integrated optical logic circuits (IOLC's) are therefore suited to operate at the interface between extremely high-speed computers and high-capacity fiber optic data networks.
To take maximum advantage of the guided wave components suited to perform the functions required at these interfaces, it is also necessary to consider the network configuration. Several alternative network configurations shown in FIGS. 1, 2 and 3 have been examined by the designers of Fibernet to provide a distributed packet switching system. R. M. Metcalfe and D. R. Boggs, "Ethernet: Distributed Packet Switching for Local Computer Networks," Comm. ACM, 19, No. 7, 395 (July 1976). These systems, however, are limited. The single-fiber bidirectional passively teed network of FIG. 1 appears to be limited by insertion splice and connector losses, making the number of stations unattractively low. Reflections at connectors, splices and tees would interfere with station monitoring or other control functions required of a distributed packet switching system.
In the unidirectional repeatered networks of FIGS. 2 and 3, active optical guided wave components are required for use in the interface to match an extremely high-speed computer capability to the high fiber bandwidth. The active data network of FIG. 2 uses separate fibers for each direction, and requires that the host computer determine whether the packet is addressed to it, or if the packet is to continue on to another destination. This decision-making capability requires processing time from the host computer, thus reducing its efficiency and the network capacity. Alternatively, as shown in FIG. 3, a unidirectional repeatered loop network has been proposed, fashioned after the Irvine Ring. Neither system has developed much interest apparently because the repeaters use active components with the usual reliability problems as well as the difficulty of supplying power to remote locations.